Message-ID: <CAJuCfpGWLnu+r2wvY2Egy2ESPD=tAVvfVvAKXUv1b+Z0hweeJg@mail.gmail.com>
Date: Fri, 1 Aug 2025 09:41:31 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Peter Xu <peterx@...hat.com>
Cc: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org, aarcange@...hat.com, 
	lokeshgidra@...gle.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	syzbot+b446dbe27035ef6bd6c2@...kaller.appspotmail.com, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/1] userfaultfd: fix a crash when UFFDIO_MOVE handles
 a THP hole

On Fri, Aug 1, 2025 at 9:23 AM Peter Xu <peterx@...hat.com> wrote:
>
> On Fri, Aug 01, 2025 at 08:28:38AM -0700, Suren Baghdasaryan wrote:
> > On Fri, Aug 1, 2025 at 7:16 AM Peter Xu <peterx@...hat.com> wrote:
> > >
> > > On Fri, Aug 01, 2025 at 09:21:30AM +0200, David Hildenbrand wrote:
> > > > On 31.07.25 17:44, Suren Baghdasaryan wrote:
> > > >
> > > > Hi!
> > > >
> > > > Did you mean in you patch description:
> > > >
> > > > "userfaultfd: fix a crash in UFFDIO_MOVE with some non-present PMDs"
> > > >
> > > > Talking about THP holes is very very confusing.
> > > >
> > > > > When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it
> > > > > encounters a non-present THP, it fails to properly recognize an unmapped
> > > >
> > > > You mean a "non-present PMD that is not a migration entry".
> > > >
> > > > > hole and tries to access a non-existent folio, resulting in
> > > > > a crash. Add a check to skip non-present THPs.
> > > >
> > > > That makes sense. The code we have after this patch is rather complicated
> > > > and hard to read.
> > > >
> > > > >
> > > > > Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> > > > > Reported-by: syzbot+b446dbe27035ef6bd6c2@...kaller.appspotmail.com
> > > > > Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/
> > > > > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > > > > Cc: stable@...r.kernel.org
> > > > > ---
> > > > > Changes since v1 [1]
> > > > > - Fixed step size calculation, per Lokesh Gidra
> > > > > - Added missing check for UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES, per Lokesh Gidra
> > > > >
> > > > > [1] https://lore.kernel.org/all/20250730170733.3829267-1-surenb@google.com/
> > > > >
> > > > >   mm/userfaultfd.c | 45 +++++++++++++++++++++++++++++----------------
> > > > >   1 file changed, 29 insertions(+), 16 deletions(-)
> > > > >
> > > > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > > > > index cbed91b09640..b5af31c22731 100644
> > > > > --- a/mm/userfaultfd.c
> > > > > +++ b/mm/userfaultfd.c
> > > > > @@ -1818,28 +1818,41 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
> > > > >             ptl = pmd_trans_huge_lock(src_pmd, src_vma);
> > > > >             if (ptl) {
> > > > > -                   /* Check if we can move the pmd without splitting it. */
> > > > > -                   if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
> > > > > -                       !pmd_none(dst_pmdval)) {
> > > > > -                           struct folio *folio = pmd_folio(*src_pmd);
> > > > > +                   if (pmd_present(*src_pmd) || is_pmd_migration_entry(*src_pmd)) {
> > >
> > > [1]
> > >
> > > > > +                           /* Check if we can move the pmd without splitting it. */
> > > > > +                           if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
> > > > > +                               !pmd_none(dst_pmdval)) {
> > > > > +                                   if (pmd_present(*src_pmd)) {
>
> [2]
>
> > > > > +                                           struct folio *folio = pmd_folio(*src_pmd);
>
> [3]
>
> > > > > +
> > > > > +                                           if (!folio || (!is_huge_zero_folio(folio) &&
> > > > > +                                                          !PageAnonExclusive(&folio->page))) {
> > > > > +                                                   spin_unlock(ptl);
> > > > > +                                                   err = -EBUSY;
> > > > > +                                                   break;
> > > > > +                                           }
> > > > > +                                   }
> > > >
> > > > ... in particular that. Is there some way to make this code simpler / easier
> > > > to read? Like moving that whole last folio-check thingy into a helper?
> > >
> > > One question might be relevant is, whether the check above [1] can be
> > > dropped.
> > >
> > > The thing is __pmd_trans_huge_lock() does double check the pmd to be !none
> > > before returning the ptl.  I didn't follow closely on the recent changes on
> > > mm side on possible new pmd swap entries, if migration is the only possible
> > > one then it looks like [1] can be avoided.
> >
> > Hi Peter,
> > is_swap_pmd() check in __pmd_trans_huge_lock() allows for (!pmd_none()
> > && !pmd_present()) PMD to pass and that's when this crash is hit.
>
> First for all, thanks for looking into the issue with Lokesh; I am still
> catching up with emails after taking weeks off.
>
> I didn't yet read into the syzbot report, but I thought the bug was about
> referencing the folio on top of a swap entry after reading your current
> patch, which has:
>
>         if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
>             !pmd_none(dst_pmdval)) {
>                 struct folio *folio = pmd_folio(*src_pmd); <----
>
> Here looks like *src_pmd can be a migration entry. Is my understanding
> correct?

Correct.

>
> > If we drop the check at [1] then the path that takes us to
>
> If my above understanding is correct, IMHO it should be [2] above that
> makes sure the reference won't happen on a swap entry, not necessarily [1]?

Yes, in case of migration entry this is what protects us.

>
> > split_huge_pmd() will bail out inside split_huge_pmd_locked() with no
> > indication that split did not happen. Afterwards we will retry
>
> So we're talking about the case where it's a swap pmd entry, right?

Hmm, my understanding is that it's being treated as a swap entry but
in reality is not. I thought THPs are always split before they get
swapped, no?

> Could you elaborate why the split would fail?

Just looking at the code, split_huge_pmd_locked() checks for
(pmd_trans_huge(*pmd) || is_pmd_migration_entry(*pmd)).
pmd_trans_huge() is false if !pmd_present() and it's not a migration
entry, so __split_huge_pmd_locked() will be skipped.

>  AFAIU, split_huge_pmd_locked()
> should still work for a migration pmd entry.

That is correct because the above mentioned is_pmd_migration_entry()
check will pass.

>
> Thanks,
>
> > thinking that PMD got split and leaving further remapping to
> > move_pages_pte() (see the comment before "continue"). I think in this
> > case it will end up in the same path again instead (infinite loop). I
> > didn't test this but from the code I think that's what would happen.
> > Does that make sense?
> >
> > >
> > > And it also looks applicable to also drop the "else" later, because in "if
> > > (ptl)" it cannot hit pmd_none().
> > >
> > > Thanks,
> > >
> > > --
> > > Peter Xu
> > >
> >
>
> --
> Peter Xu
>

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